A vacuum booster is widely known to generate an assist force for assisting a braking operation force produced by a driver and applied to a master cylinder. By using a pressure lower than an atmospheric pressure (hereinafter referred to as a “negative pressure”) such as a manifold air pressure of an internal combustion engine, the braking operation by the driver is assisted according to the known vacuum booster. Such assist will be hereinafter referred to as a “vacuum assist”.
In addition, a brake hydraulic pressure control apparatus is widely known to have a hydraulic pump for generating an assist hydraulic pressure added to a hydraulic pressure generated by the master cylinder, and a solenoid valve for adjusting and controlling the assist hydraulic pressure. By using the assist hydraulic pressure, the braking operation by the driver is assisted according to the known brake hydraulic pressure control apparatus. Such assist will be hereinafter referred to as a “pump assist”.
JP9030385A and JP9323641A disclose apparatuses each assisting a braking operation performed by the driver by means of the pump assist in addition to the vacuum assist. Such apparatuses will be explained with reference to FIG. 21.
FIG. 21 is a graph illustrating characteristics of a braking pressure (i.e., wheel cylinder pressure) relative to a braking operation force F produced by the driver. In FIG. 21, a characteristic line Ch1 shows characteristics in cases where neither the vacuum assist nor the pump assist is performed. A characteristic line Ch2 shows characteristics in cases where only the vacuum assist is performed. A change point (i.e., vacuum assist limit point) on the characteristic line Ch2 is a point where an application of the braking pressure by the vacuum assist reaches a limit value (i.e., maximum value).
A characteristic line Ch3 shows characteristics according to the apparatus disclosed in JP9030385A. As illustrated by the characteristic line Ch3, only the vacuum assist is performed until the braking operation force F reaches a value FB0 that corresponds to the change point (F≦FB0). In cases where the braking operation force F reaches the value FB0, the pump assist is then started. Accordingly, the decrease in a rising gradient of the braking pressure relative to the increase in the braking operation force F within an area where the braking operation force F exceeds the value FB0 can be compensated.
A characteristic line Ch4 shows characteristics according to the apparatus disclosed in JP9323641A. As illustrated by the characteristic line Ch4, only the vacuum assist is generally performed. In cases where a deceleration speed of a wheel exceeds a predetermined value because of a sudden braking, and the like, the pump assist is started and performed relative to the braking operation force F at that time (=value F1). Accordingly, a vehicle can be securely rapidly accelerated at a time of the sudden braking, and the like.
The pump assist is achieved by a hydraulic pressure control based on an electronic control of a motor for driving a hydraulic pump, a solenoid valve, and the like. On the other hand, the vacuum assist is achieved by only a mechanical structure of the vacuum booster, without the use of the electronic control. Thus, dynamic characteristics in application of the braking pressure between the pump assist and the vacuum assist relative to the braking operation force F are inevitably different because of a difference in operating principles between the pump assist and the vacuum assist.
Accordingly, when the pump assist is started during the braking operation during which only the vacuum assist is performed, the driver may tend to have an uncomfortable feeling in a brake operation. Such tendency is most pronounced in cases where the pump assist is started at a time when the braking operation force F becomes sufficiently large (i.e., at a time when the vacuum assist is sufficiently large) as in the apparatuses disclosed in JP9030385A and JP9323641A.
A need thus exists for a braking control apparatus for a vehicle which is not susceptible to the drawback mentioned above.